46,838 research outputs found
How education shaped communist Cuba
F is for Fidel, Y is for Yanqui. This mantra used for teaching the alphabet in revolutionary Cuba shows just how far its educational divide with the U.S. has stretched. No sector illustrates better how Cuba and the U.S. have grown apart in over 50 years than education. Cuba claims today that its academic standards are among the highest in the world, and the country has educated tens of thousands of foreign students, mostly in medicine. U.S. policymakers know little about the methods used in Cuban education, nor what practical opportunities for collaboration in research and business might exist. With the agreement the two countries made last December to restore diplomatic relations, that may be about to change
Very accurate Distances and Radii of Open Cluster Cepheids from a Near-Infrared Surface Brightness Technique
We have obtained the radii and distances of 16 galactic Cepheids supposed to
be members in open clusters or associations using the new optical and
near-infrared calibrations of the surface brightness (Barnes-Evans) method
given by Fouque & Gieren (1997). We discuss in detail possible systematic
errors in our infrared solutions and conclude that the typical total
uncertainty of the infrared distance and radius of a Cepheid is about 3 percent
in both infrared solutions, provided that the data are of excellent quality and
that the amplitude of the color curve used in the solution is larger than ~0.3
mag.
We compare the adopted infrared distances of the Cepheid variables to the
ZAMS-fitting distances of their supposed host clusters and associations and
find an unweighted mean value of the distance ratio of 1.02 +- 0.04. A detailed
discussion of the individual Cepheids shows that the uncertainty of the
ZAMS-fitting distances varies considerably from cluster to cluster. We find
clear evidence that four Cepheids are not cluster members (SZ Tau, T Mon, U Car
and SV Vul) while we confirm cluster membership for V Cen and BB Sgr for which
the former evidence for cluster membership was only weak. After rejection of
non-members, we find a weighted mean distance ratio of 0.969 +- 0.014, with a
standard deviation of 0.05, which demonstrates that both distance indicators
are accurate to better than 5%, including systematic errors, and that there is
excellent agreement between both distance scales.Comment: LaTeX, 11 Figures, 5 Tables, to be published in The Astrophysical
Journal, Oct. 10, 1997 issu
Energy spectrum, dissipation and spatial structures in reduced Hall magnetohydrodynamic
We analyze the effect of the Hall term in the magnetohydrodynamic turbulence
under a strong externally supported magnetic field, seeing how this changes the
energy cascade, the characteristic scales of the flow and the dynamics of
global magnitudes, with particular interest in the dissipation.
Numerical simulations of freely evolving three-dimensional reduced
magnetohydrodynamics (RHMHD) are performed, for different values of the Hall
parameter (the ratio of the ion skin depth to the macroscopic scale of the
turbulence) controlling the impact of the Hall term. The Hall effect modifies
the transfer of energy across scales, slowing down the transfer of energy from
the large scales up to the Hall scale (ion skin depth) and carrying faster the
energy from the Hall scale to smaller scales. The final outcome is an effective
shift of the dissipation scale to larger scales but also a development of
smaller scales. Current sheets (fundamental structures for energy dissipation)
are affected in two ways by increasing the Hall effect, with a widening but at
the same time generating an internal structure within them. In the case where
the Hall term is sufficiently intense, the current sheet is fully delocalized.
The effect appears to reduce impulsive effects in the flow, making it less
intermittent.Comment: 17 pages, 10 figure
A precessing jet model for the PN K 3-35: simulated radio-continuum emission
The bipolar morphology of the planetary nebula (PN) K 3-35 observed in
radio-continuum images was modelled with 3D hydrodynamic simulations with the
adaptive grid code yguazu-a. We find that the observed morphology of this PN
can be reproduced considering a precessing jet evolving in a dense AGB
circumstellar medium, given by a mass loss rate
\dot{M}_{csm}=5x10^{-5}M_{\odot}/yr and a terminal velocity v_{w}=10 km/s.
Synthetic thermal radio-continuum maps were generated from numerical results
for several frequencies. Comparing the maps and the total fluxes obtained from
the simulations with the observational results, we find that a model of
precessing dense jets, where each jet injects material into the surrounding CSM
at a rate \dot{M}_j=2.8x10^{-4} {M_{\odot}/yr (equivalent to a density of
8x10^{4} {cm}^{-3}, a velocity of 1500 km/s, a precession period of 100 yr, and
a semi-aperture precession angle of 20 degrees agrees well with the
observations.Comment: 6 pages, 4 figures, accepted to MNRA
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